A supraparticle-based biomimetic cascade catalyst for continuous flow reaction

Robust millimeter-sized spherical particles with controlled compositions and microstructures hold promises of important practical applications especially in relation to continuous flow cascade catalysis. However, the efficient fabrication methods for producing such particles remain scare. Here, we demonstrate a liquid marble approach to fabricate robust mm-sized porous supraparticles (SPs) through the bottom-up assembly of silica nanoparticles in the presence of strength additive or surface interactions, without the need for the specific liquid-repellent surfaces used by the existing methods. As the proof of the concept, our method was exemplified by fabricating biomimetic cascade catalysts through assembly of two types of well-defined catalytically active nanoparticles. The obtained SP-based cascade catalysts work well in industrially preferred fixed-bed reactors, exhibiting excellent catalysis efficiency, controlled reaction kinetics, high enantioselectivity (99% ee) and outstanding stability (200~500 h) in the cascades of ketone hydrogenation-kinetic resolution and amine racemization-kinetic resolution. The excellent catalytic performances are attributed to the structural features, reconciling close proximity of different catalytic sites and their sufficient spatial isolation.

Notes：The average diameter of MSNs is ca. 100 nm, measured by TEM. The BET specific surface area was measured to be 741 m 2 g −1 and the average pore size is ca. 5.7 nm, calculated from the BJH method.

Notes:
To check whether the flow process leads to the leaching of CALB from Pd-CALB/SPs, we examined the catalytic activity of the outflow by adding another substrates (4-methyl-2-pentanol) instead of previous 1-phenylamine. As shown above, no new products were determined even during 12 h of reaction, indicating no significant CALB leaching from the fixed-bed reactor. Moreover, a protein assay was also performed to further examine the CALB leaching. The result was determined to be below the detection limit.

Characterization
Transmission  and were subjected to be fluidized with N2 at a superficial velocity of 0.3 m s -1 for 24 h. Likewise, 0.5 g SPs were directly packed in a fixed-bed reactor and treated with flowing n-octane (flow rate: 6 mL h -1 ) at 4 MPa N2 (50 mL min -1 ) for 48 h. After the treatment, the sample was collected for SEM observation and TG test.

Fluorescent permeability tests.
SPs were deposited on a glass slide. for all the reactions) were then added into the above solution, respectively. Before reaction, the autoclave was sealed and flushed with H2 five times to remove the air.
After being charged with a 2 MPa H2 at room temperature, the autoclave was then heated to 50 o C while being stirred (600 rpm) for reaction to occur. The autoclave was allowed to cool to room temperature after reaction. The products were analyzed by GC at intervals and further confirmed with GC-MS. The catalyst was isolated from the reaction mixture via filtration, thoroughly washed with n-octane five times, and then dried at 50 o C under a vacuum, ready to be used in the next reaction cycle.
Sequential hydrogenation-kinetic resolution over Pd-CALB/SPs in continuousflow system. In a typical reaction, 0.85 g Pd-CALB/SPs (0.050 mmol Pd; 19.8 mg CALB) and 0.2 g CALB/SPs (9.3 mg CALB) were mixed with quartz sand (120-160 S57 mesh) to pack in a fixed-bed, at bottom and top of which were filled with quartz sand (40-60 mesh). Before reaction, the catalyst was flushed with H2 at 2 MPa to remove air.
Subsequently, a solution of ketone (0.1 M), vinyl acetate (0.4 M) and dodecane (internal standard) in n-octane as mobile phase was pumped through the inlet of the column reactor at a given flow rate and was allowed to pass through the column reactor whose temperature was kept at 50 o C. The outflow of the column reactor was sampled for GC analysis at regular intervals. The product was further confirmed with GC-MS. after 500 h.) and was allowed to pass through the column reactor whose temperature was kept at 70 o C. After running for 500 h, about 466 mL of the amine solution was used. The outflow of the column reactor was sampled for GC analysis at regular intervals. The product was further confirmed with GC-MS.